1. Field of the Invention
The invention relates to a chiral nematic liquid crystal composition, a liquid crystal display device, and the production method thereof.
2. Description of the Related Art
Liquid crystal display devices employing a chiral nematic liquid crystal composition containing an added chiral agent in its nematic liquid crystal that is in the cholesteric phase at room temperature have been known. Such liquid crystal display devices fundamentally have a chiral nematic liquid crystal composition put and held between a pair of substrates having a transparent electrode, and the display is performed by switching the liquid crystal between its planar (PL) state and the focal conic (FC) or homeotropic (Homeo) state while applying a high/low pulse voltage (driving voltage) between the electrodes. For example, the display is “bright” in the PL state because the light having a particular peak wavelength is selectively reflected and “dark” in the FC or Homeo state because the light is transmitted.
The liquid crystal display devices are generally produced by enclosing a liquid crystal composition between substrates at room temperature. In particular, when the fluidity of a liquid crystal composition is lower, the liquid crystal composition is known to be first heated for increase in fluidity and enclosed between substrates.
However, the contrast between the “bright” and “dark” states was not sufficiently high in the display devices above. Improvement in the visibility of display image is required.
On the other hand, a liquid crystal composition containing a ferroelectric liquid crystal (SCE8; phase transition temperature between liquid crystal and isotropic phases=102° C.) and a self-organizing type gelling agent that has a phase transition temperature between liquid crystal and isotropic phases higher than the sol-gel transition temperature of the gelling agent was disclosed (Liquid Crystal”, Hiroshi Kato, published by the Japanese Liquid Crystal Society, 2000, Vol. 4, No. 1, pp. 17 to 18). Specifically, in the liquid crystal composition, the phase transition temperature between liquid crystal and isotropic phases is set to a temperature higher than the sol-gel transition temperature of the gelling agent; and in production of the liquid crystal display device, the liquid crystal composition is cooled down from a heated state to room temperature, while strictly controlling the conditions such as “rubbing during cell production” and “application of voltage after injection of liquid crystal”. In this manner, the isotropic phase is converted to a liquid crystal phase (FIG. 6(A): isotropic phase, FIG. 6(B): liquid crystal phase), and the orientation of the liquid crystal molecules serves as a template and generates anisotropic linear orientation of the gelling agent molecules (FIG. 6(C): orientation of gelling agent molecules (black region)). Although the literature above describes only generation of a new type of molecular self-organization, it seems to be also effective for improving the shock-resistance and other properties of ferroelectric liquid crystal panels.